1. Technical Field
The present disclosure generally relates to seismic prospecting and in particular to methods and apparatus for generating seismic source signals with reduced signal distortions.
2. Background Information
In the oil and gas exploration industry, geophysical tools and techniques are commonly employed in order to identify a subterranean structure having potential hydrocarbon deposits. Many different techniques are used to generate a seismic signal.
Seismic vibratory energy sources have been used in the field many years. A seismic vibrator in its simplest form is merely a heavy vehicle that has the ability to shake the ground at a predetermined range of frequencies of about 2 to 250 Hz. The vibrator imparts a signal into the subsurface of the earth over a relatively long period of time, which allows for an energy level less than impulse generators such as dynamite.
The imparted energy, known as the seismic source signal or “pilot” signal, travels through the subsurface and reflects some of the energy from certain subsurface geological boundaries or layers. The reflected energy is then transmitted back to the earth's surface where it is recorded using an earth motion detector. The recorded data is processed to yield information about a location and physical properties of layers making up the subsurface.
The seismic vibrator source signal is typically a sweep signal, or simply sweep. Sweeps are sinusoidal vibrations in the 2-250 Hz range described above and may have a duration on the order of 2 to 20 seconds depending on the terrain, the subsurface lithology, economic constraints and physical capabilities of the vibrator. The sinusoidal sweep can be increased in frequency overtime, which is called an “upsweep”. The upsweep is the signal used typically in modern seismic exploration. Also, the sinusoidal sweep can be decreased in frequency overtime, which is called a “downsweep”. The end products of the vibrator sweep are waves that propagate through the earth to return clues about the subsurface.
A problem with the typical sweep is that the signal imparted into the earth includes distortions caused by harmonic signals generated by one or more of the seismic source components, e.g. the hydraulic, mechanical and electromechanical subsystems making up the source.